Although the invention will be explained in the following based on an oxygen sensor working according to the principle of fluorescence quenching, the inventive idea is not limited to such sensors. Rather, other process variables, especially concentrations of certain analytes, such as ions, molecules, gases or other chemical compounds, pH-value or temperature are likewise measurable by such an arrangement with corresponding modifications. Measuring devices suitable for determining such process variables are manufactured and sold by the group of firms, Endress+Hauser, in a large number of variants.
The sensor includes, for instance, a sensor head containing an optical sensor element. Adjoining the sensor head is a housing, which contains a data processing unit, wherein the optical sensor element is irradiated with light from a light source. The light, possibly after first being converted, is radiated back by the optical sensor element with a certain light characteristic, detected by a light receiver and a signal of the light receiver representing the light characteristic evaluated by the data processing unit.
Known from European Patent EP 2 295 953 A1 is a system for measuring substance concentrations in solutions based on a fluorescence measurement. The system includes a light source, which radiates light into a medium to be examined. This transmitted light excites an optical sensor element, which is arranged in contact with the medium to be examined. In the fluorescence measurement, the transmitted light is absorbed by the optical sensor element and light of another wavelength radiated back as a function of a process variable, thus, for instance, the concentration of an analyte. The radiation radiated back by the optical sensor element is absorbed by a light receiver as received light, converted into an electrical, measured variable and forwarded to a data processing unit. Depending on properties of the optical sensor element, the optical sensor reacts to different particle concentrations with different received light intensities, received frequencies, phase angle and/or decay curves.
Fundamentally, there are different methods for arranging the light source/light receiver relative to the optical sensor element, as is explained below.
In the case of sensors with sufficient energy supply, the light source/light receiver can be arranged directly at the optical sensor element. This is, however, difficult to implement in the case of high temperature sensors, since long and disturbance susceptible, connecting lines result between the optical components and the data processing unit arranged remotely from the high temperature measuring point.
In order to avoid this, light source and light receiver can be placed far from the possibly hot medium to be examined. Then the light can be brought to the optical sensor element via a single light conductor. This cannot be done for compact sensors characterized by small constructions, since a relatively large light conductor is required.
Additionally, separate light conductors can be used for light source and light receiver. This is known, for example, from European Patent EP 0 940 662 B1. For an optimal measurement signal, the light conductor must point at the optical sensor element at an angle of, for instance, 45°. This again cannot be implemented for compact sensors.
Described in German Patent DE 102 18 606 A1 is a digital sensor, which is composed of two components connected releasably with one another: A sensor-side component (plug head), with which a sensor element (there, a potentiometric sensor) and a data memory are inseparably connected, and a transmitter-side component (a pluggable connector coupling or a sensor cable), via which the sensor-side component is coupled with a measurement transmitter or directly with a control system. A digital, bidirectional data transfer between the two described sides occurs contactlessly via a magneto inductively coupling interface. Energy transfer via the contactless, magneto inductive interface occurs unidirectionally. Energy transmission via a galvanically separated interface provides comparatively little energy to the sensor side component. Corresponding sensors are manufactured and sold by the applicant under the mark “Memosens”. The “Memosens” technology is applicable not only for potentiometric sensors, but, instead, in principle, for any sensor used for determining and monitoring the most varied of process variables.